There are many kinds of AC voltage sources. AC voltage sources of 110 and 220 volts are the most commonly used type of voltage sources. To permit use with the different voltage sources, electric household appliances should accommodate different AC voltages. In general, a preset switch is installed on the power supply for this purpose. In order to accommodate the different voltages in various areas, the makers of appliances have previously been required to manually preset the switch in an appropriate position, because if the switch is wrongly preset, damage to the appliances will result. Appliances damaged in this manner cannot be reused, and the consequential damage resulting from such a mistake can be disastrous. To solve this problem, a device which not only can automatically sense an AC input voltage and also automatically preset the voltage preset switch to an appropriate position, is desired by the people.
A commonly used AC voltage sensor circuit is shown in FIG. 1. It consists of a transformer 101, a rectifier 102, a filter 103, a voltage stabilizer 104 and an operational amplifier 110 which acts as a comparator. The comparator circuit (operational amplifier 110) should be operated at a DC voltage of less than 30 volts and, consequently, the voltage of the input AC power must first be changed by transformer 101, then rectified by rectifier diode 102 and filtered by a filter capacitor to provide the necessary DC power. The voltage stabilizer 104 produces stabilized DC power. The stabilized DC power can be used as operational power for the operational amplifier 110. On the other hand, the DC power is also divided by the resistors 108 and 109 to function as reference voltages. Capacitor 106 is used as a filter and voltage stabilizer. The voltage on the filter capacitor 103 is changed according to the input AC voltage. After it is divided by resistors 106 and 107, and compared with a reference voltage, by means of the output voltage of the operational amplifier 110, the input AC voltage can be identified. Since this device requires a transformer, DC stabilizer, and operational amplifier, the total weight and the cost are very high.